"TypeERROR" in animation nodes2.1
The CustomFalloff expects a FloatList object instead of a normal python list. So you should import the FloatList from data_structures:

from ... data_structures import FloatList


Then in the execute function do something like:

strengths = FloatList(iterations)
for i in range(iterations):
        strengths[i] = factor * i

return stepFalloff(strengths, fallback)